1. Technical Field of the Invention
The present invention relates to piezoelectric resonators and methods for adjusting resonant frequencies thereof, and more particularly, to a piezoelectric resonator which maximizes the use of a mechanical resonance of a piezoelectric member and electronic components including such a piezoelectric resonator, such as an oscillator, a discriminator, and a filter, and methods of adjusting resonant frequencies of piezoelectric resonators contained in such electronic components.
2. Description of the Related Art
In a conventional piezoelectric resonator, electrodes are located on both surfaces of a piezoelectric substrate which has a rectangular plate shape or a square plate shape as viewed from above. The piezoelectric substrate is polarized in the thickness direction. When a signal is input between the electrodes, an electric field is applied to the piezoelectric substrate in the thickness direction and the piezoelectric substrate vibrates in a direction which is parallel to the major surfaces thereof.
The piezoelectric resonator described above is an unstiffened type, in which the vibration direction differs from the direction of polarization and the direction of application of electric field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. Therefore, an unstiffened piezoelectric resonator has a relatively small frequency difference .increment.F between the resonant frequency and the antiresonant frequency. This causes a frequency bandwidth in use to be undesirably narrow when an unstiffened frequency resonator is used as an oscillator or a filter. Therefore, the degree of freedom in resonator characteristics and filter design is low in such a piezoelectric resonator and electronic components including such a piezoelectric resonator.
The piezoelectric resonator described above, which uses the piezoelectric substrate having a rectangular plate shape as viewed from above, uses a first-order resonance in the longitudinal mode. As a result of its structure, it also generates large spurious resonances in odd-order harmonic modes, such as the third-order and fifth-order modes, and in the width mode.
The piezoelectric resonator described above, which uses the piezoelectric substrate having a square plate shape as viewed from above, uses the first-order resonance in the longitudinal mode. As a result of its structure, this resonator is also likely to generate large spurious resonances, such as in the thickness mode, and to generate a third harmonic in the square vibration mode.
There has been proposed a laminated piezoelectric resonator having small spurious resonance and a large difference .increment.F between the resonant frequency and the antiresonant frequency. FIG. 20 is a view of such a laminated piezoelectric resonator. In the laminated piezoelectric resonator 1 shown in FIG. 20, a plurality of piezoelectric layers 3 and a plurality of inner electrodes 4 are alternately laminated to form a narrow base member 2, and each of the plurality of piezoelectric layers 3 is polarized in the longitudinal direction of the base member 2. This laminated piezoelectric resonator 1 is a stiffened type, and includes piezoelectric layers 3 in which the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electric field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference .increment.F between the resonant frequency and the antiresonant frequency. In addition, since this laminated piezoelectric resonator 1 is a stiffened type, vibrations in modes such as the width mode and the thickness mode, which are different from the fundamental vibration, are unlikely to occur. In this laminated piezoelectric resonator 1, the edges of the inner electrodes 4 are exposed at all side surfaces of the base member 2. At one side surface of the base member 2, an insulating film 5a covers one end of an edge of each alternate inner electrode 4, and an external electrode 6a is formed so as to connect to the other alternate inner electrodes 4. On the side surface of the base member 2, in order for an external electrode 6b to be connected to the alternate inner electrodes 4 for which the insulating film 5a is formed, an insulating film 5b covers the other end of an edge of each of the other alternate inner electrodes 4 and then, an external electrode 6b is formed.
When the laminated piezoelectric resonator 1 shown in FIG. 20 is mass-produced, the desired resonant frequency and the desired antiresonant frequency cannot be obtained in some cases due to manufacturing variations. If the frequency of the piezoelectric resonator 1 is lower than the desired frequency, the frequency can be increased to the desired frequency by cutting off a portion of the resonator at an end thereof such that the total length of the resonator is decreased. If the frequency of the piezoelectric resonator 1 is higher than the desired frequency, however, the frequency cannot be reduced to the desired frequency. Therefore, manufacturing yield is low.